Not sure about Germany, but in Austin, Uber wanted their drivers to carry auto insurance, although the company also has its own policy to cover drivers--many personal auto insurance policies explicitly do not cover accidents while the insured party is driving for hire.

About as well as you think, which is zero. If an insurance company finds out your injured/killed passenger was for-hire under the table, most standard policies don't cover this especially in the age of Uber. You're screwed.